This invention relates generally to magnetic resonance imaging magnets and more particularly concerns an open, frustoconical-shaped magnet arrangement which uses a plurality of refrigerated superconducting coils.
Magnetic resonance imaging (MRI) is now a widely accepted medical diagnostic procedure and its use is becoming increasingly popular. MRI systems require a uniform magnetic field and radio frequency radiation to create an MR image. Various types of magnets are currently used to produce the magnetic field. Whatever type of magnet is used, it is necessary that a portion of the magnetic field be highly homogeneous. This portion of the magnetic field, referred to herein as the imaging volume, is the portion of the field which covers the subject area being imaged. Field homogeneity in the imaging volume is necessary to obtain a quality image. Therefore, it is advantageous for many imaging applications to have a magnet which can produce a relatively large imaging volume.
Currently, the magnets most commonly used for whole body imaging are arranged so as to be contained within a long cylindrical structure having a relatively narrow central bore. The narrow bore forms an enclosed chamber into which a patient must enter for imaging. Typically, these structures are approximately 2.4 meters in length with a bore diameter of about one meter. These cylindrical arrangements provide many positive features, but there are some circumstances in which they present difficulties. For example, a significant percentage of patients are sensitive to the closed nature of conventional MRI systems. For them, the MRI process can be uncomfortable or even unbearable. Furthermore, some patients are simply too large to fit into the narrow bore of a conventional MRI system. Difficulties can also arise in placing a patient in the enclosed chamber if the patient is required to remain connected to an IV system or other medical equipment. The closed conventional systems are also difficult for veterinary applications because many animals are frightened by the closed chamber.
Various open MRI magnets have been proposed as more accessible alternatives to the closed conventional systems. For instance, U.S. Pat. No. 4,875,485, issued Oct. 24, 1989 to Kinya Matsutani and U.S. Pat. No. 4,924,198 to Evangelos T. Laskaris both show magnet assemblies having two superconducting coils arranged in a spaced-apart, parallel relationship. The open space between the two coils is well-suited to receiving patients and is not likely to induce claustrophobic reactions. While these arrangements are effective, the homogeneity could be better. Thus, it would be advantageous to have an MRI device which provides a more open feeling than conventional systems and produces a large imaging volume having an exceptional level of homogeneity.